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74HC393 Binary Counter

Discussion in 'General Electronics Discussion' started by AmandaM, May 24, 2015.

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  1. AmandaM

    AmandaM

    21
    3
    May 15, 2015
    Hello Everyone

    I am currently experimenting with the 74HC393 ripple binary counter. Every time a reed switch activates it causes the 74HC393 to increase its count. After playing for some time and discovering lots of false positives I realised that I needed to debounce the reed switch before it got to the IC counter. With that in mind, I built a bouncing circuit using a RC bridge coupled with a 74HC14 Schmitt Trigger and now the counter is counting perfectly.

    However, I've a new problem. I would like to use the master reset function, the datasheet says that if I make the reset pin active HIGH then everything should reset. That is, all outputs return to zero. I have tried making this pin momentarily high by touching the pin with 5v but this seems to short out my circuit and of course my microprocessors reset do to power loss.

    I have checked and double checked all connections and there is nothing obvious which could cause a short so I guess that it must be something internal in the 74HC393.

    Any idea what I am doing wrong. I can post a schematic if necessary,

    PS: Not sure if it is relevant but I am only using two bits of the counter, the others are left floating. All unused inputs are routed to ground.

    Once again, I welcome all comments and thank people for helping me with my beginner questions.

    Thanks
    Amanda
     
  2. chopnhack

    chopnhack

    1,573
    352
    Apr 28, 2014
    Post the schematic - it will enable quick troubleshooting. Your question is clear, but we can't see what you are working on without it!
     
    AmandaM likes this.
  3. AmandaM

    AmandaM

    21
    3
    May 15, 2015
    Hi

    I have finished putting together a basic schematic of my circuit, please excuse its crudity, this is actually the first electronic schematic that I have ever put together. Still very new with electronics but am a seasoned programmer. Please find said attached to this post.

    • The switch in the schematic is actually a reed-switch from a rain gauge. Each tip of the rain gauge sends a quick pulse (Maybe around 100ms). As previously stated, this switch is very bouncy.
    • The schematic also shows my RC debounce circuit who's output goes straight to the Schmitt trigger.
    • Output of Schmitt trigger goes to the SET pin of the binary counter.
    • Test LED's are connected to two of the counters output pins
    • The counters output pin is pulled up to 5v
    • Unused inputs are tied to ground
    As I said in my original post, the counter functionality is working perfectly. The first activation of the rain gauge lights LED1, the second lights LED2 and the third lights both. Obviously, the next activation switches both LED's off. Eventually, I will use a ATMega1284P to reed the voltages on the output pins, add to a counter variable and then send HIGH to the RESET pin and reset the counter regardless of where it is in its cycle.

    Therein lies my problem. The datasheet says that if I make the reset pin (1MR in my schematic) active high then the counter should reset all outputs to zero. However, if I make 1MR high then it causes a short and everything resets. I am obviously doing something wrong.
    Debounce.png

    I appreciate any help and all comments.

    Many Thanks
    Amanda

    EDIT:

    I don't think it makes any difference to my problem but I forgot to add the filtering capacitors to the input and output of my 7805 regular in the attached schematic.
     
    Last edited: May 24, 2015
  4. chopnhack

    chopnhack

    1,573
    352
    Apr 28, 2014
    What are you using to bring mclr high? A momentary switch? Are you buffering it with a resistor? I don't believe you can directly connect Vdd to the pin, overcurrent would occur and the μcu shuts down to prevent damage. I have a recent post that shows a schematic with circuitry for mclr pin on a PIC. Should be similar. Hope this helps. :)
     
  5. AmandaM

    AmandaM

    21
    3
    May 15, 2015
    Thanks Chopnhack

    For testing purposes I am simply touching 5v to the clear pin. Eventually, a Microcontroller will send a HIGH signal based on what it finds on the output pins. I have tried buffering it with a resistor, the circuit did not short but there was no reset either. I will find your schematic and see if it helps.

    Many thanks
    Nicola
     
  6. AmandaM

    AmandaM

    21
    3
    May 15, 2015

    What was the title of your recent post? I have looked though your posts but there are lots of them ;-)
     
  7. AmandaM

    AmandaM

    21
    3
    May 15, 2015
    UPDATE:

    It seems that my circuit is working fine. As a last resort I tried replacing the counter IC and suddenly it worked perfectly. Must have been a fried chip!

    Because I am still learning the basics of electronics I would be interested in peoples comments about my circuit above. I know it works but does it look reasonable? Any improvements?

    PS: I have added a pulldown resistor to the RESET pin.

    Many thanks
    Amanda
     
    chopnhack likes this.
  8. chopnhack

    chopnhack

    1,573
    352
    Apr 28, 2014
    Sorry about that I was posting from the mobile and its not easy to search/copy/links - or at least I haven't learned how to do so easily yet. I am glad you figured out your problem! I updated my schematic with some labels to show you where the MCLR is in the circuit. See post #15

    That's good! Keep if from floating and potentially giving you odd problems in the future.

    Are there two people using your account? Amanda and Nicola?
     
    AmandaM likes this.
  9. AmandaM

    AmandaM

    21
    3
    May 15, 2015
    Hehehe, No, My first name is Nicola, my second Amanda. I wanted to keep these posts from indexing into Google with my first name but habit caused me to fail. I really should use some imagination and invent a good handle...

    Thanks for the link....
     
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